Ec2 module: add support to boto 2.5.0-

Add support to Boto library lower than 2.5.0 for ec2 module. 
Differences between Boto versions:
* If Boto is lower than 2.3.0: "virtualization_type" is always null in return object
* If Boto is lower than 2.5.0: "instance_profile_name" is not supported and raise an error if set. This parameter has been added to Ansible 1.3